(1) Technical Field
The present invention relates generally to a heat exchanger, and more specifically to a “direct-fired” or “indirect-fired” boiler for generating steam, hot water, hot oil, and hot molten metals.
(2) Related Art
All boilers operate according to the physical sciences of thermodynamics and heat transfer. Essentially, forced hot gas is cooled within the boiler by transferring heat to a heat transfer medium, often water, to generate steam or hot water. Depending upon system requirements, direct-fired boilers and/or indirect-fired boilers are commonly placed in service to produce steam and hot water. In the case of a direct-fired boiler, a fueled burner or combustor is fired into the boiler, generating heat within the boiler itself. The fueled burner establishes a flame, producing a hot fluid, which is in heat transfer relation with a cooler heat transfer medium. A temperature differential between the hot fluid and the heat transfer medium drives the heat transfer process by way of conduction, convection, and radiation.
In a similar manner, a “waste heat recovery” or indirect-fired boiler makes use of residual heat from an isolated thermodynamic process. However, radiation heat transfer is a less significant heat transfer mechanism for the indirect-fired boiler. For boilers of either direct-fired or indirect-fired construction, the heat transfer medium is usually water and/or steam, due in large part to their widespread availability and substantial heat capacity. Another advantage of water/steam heat transfer media is that it presents no imminent environmental threat.
A conventional type of direct-fired boiler, commonly called a “firetube” boiler, employs a fueled burner to generate heat. The burner is fired into a single main tube, called the firetube. This firetube absorbs the majority of the radiation emitted from the combustion process. In addition, convective/conductive couples drive heat transfer between the hot fluid and the heat transfer medium throughout the device. Conventional firetube boilers typically contain one to three additional banks of significantly smaller tubes, called passes. For example, a firetube boiler design that includes two banks of tubes in addition to the firetube is termed a “three-pass firetube boiler,” elicited from the path of the hot fluid. The course of flow for the “three-pass firetube boiler” occurs after the fueled burner generates hot gas inside the firetube, which is then driven through a first bank of smaller tubes flowing opposite the firetube, and then diverted through a second bank of smaller tubes flowing parallel to the firetube. A channel, called the “turn-around pass,” is located between each pass, wherein the hot gas reverses direction. The hot gas cools while flowing through the tube passes of the firetube boiler by transferring energy to the heat transfer medium. For either design, all tube banks, less the “turn-around pass,” are in heat transfer relationship with the heat transfer medium. In a similar manner, although a “waste heat recovery” or indirect-fired boiler does not require a firetube, the hot gas does flow sequentially from tube bank to tube bank as required to enact the heat transfer. As a result, heat transfer to the heat transfer medium is largely dependent upon the total length of the tubes it contacts. This can result in larger and more expensive devices.
Accordingly, a need exists for a heat exchange device capable of greater efficiency in the transfer of heat from its fluid to its heat transfer medium.